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The naturally-occurring estrogens are estrone (E₁, ), estradiol (E₂, ), and estriol (E₃, ). 16α-hydroxy Estrone (16α-OHE₁) is a hydroxylated metabolite of E₁ as well as an interconversion product with E₂. E₁ is 16α-hydroxylated by cytochrome P450 (CYP) isoforms, including CYP1A1, CYP3A5, CYP3A4, and CYP3A7, with CYP3A5 being breast-specific. 16α-OHE₁ is sulphatized or glucuronidated before excretion. It is increased in rheumatoid arthritis and decreased by physical activity. Unlike the parent estrogens and other hydroxylated metabolites of E₁, 16α-OHE₁ binds covalently and persistently activates estrogen receptors. In addition, this metabolite increases cell proliferation and does not suppress TNF-α secretion, whereas other estrogen metabolites are not pro-proliferative and have marked effects on TNF-α secretion. The levels of 16α-OHE₁ are increased in some forms of hormone therapy. Because hormone therapy increases breast cancer risk, 16α-OHE₁ has been implicated as a risk factor for breast cancer, although supportive data remains elusive.

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16α-Hydroxy Estrone (Estriol EP Impurity H) is a major metabolite of Estradiol.

A major metabolite of Estradiol

ChEBI: The 16alpha-hydroxy derivative of estrone; a minor estrogen metabolite.